When a circuit is powered by a voltage source, there will often be adverse effects if the voltage source is connected with reverse polarity. For example, a vehicle has positive and negative battery cables that connect to positive and negative terminals of a battery. If the positive cable is attached to the negative terminal of the battery and the negative cable is attached to the positive terminal of the battery, the voltage source is connected with reverse polarity.
Referring now to FIG. 1A, an electrical schematic of a circuit 100 incorporating a fuse-based reverse polarity protection system according to the prior art is presented. This circuit includes a transistor 102. In this implementation the transistor 102 is an n-channel metal-oxide semiconductor field-effect transistor (MOSFET) that has a gate, source, drain, and body, although other transistor types may be used. A first terminal of a voltage source 104 communicates with a first terminal of a fuse 106. A second terminal of the fuse 106 communicates with a negative terminal of a first diode 108 and with a first terminal of an inductor 110. A positive terminal of the first diode 108 communicates with a second terminal of the voltage source 104. A second terminal of the inductor 110 communicates with the drain of the transistor 102 and with a positive terminal of a second diode 112. The source and the body of the transistor 102 communicate with the second terminal of the voltage source 104. A negative terminal of the second diode 112 communicates with a first terminal of a capacitor 114. A second terminal of the capacitor 114 communicates with the second terminal of the voltage source 104.
In normal operation, voltage at the first terminal of the voltage source 104 is positive in reference to the second terminal of the voltage source 104. The fuse 106 operates as a small resistance, and the first diode 108 is reverse biased. Current flows through the inductor 110 and then into the transistor 102 and/or the second diode 112. One of the disadvantages of this approach is that current must always flow through the fuse 106, regardless of whether it later flows through the transistor 102 or through the second diode 112. The fuse must be rated for this continuous current, and dissipates power continuously.
Referring now to FIG. 1B, an electrical schematic 120 depicts the circuit of FIG. 1A when the voltage source is connected with reverse polarity. To make operation more apparent, the circuit has been redrawn upside down. The body of the transistor 102 is now at the highest potential in the circuit. The pn junction between the body of the transistor 102 and the source of the transistor 102 is now forward-biased. Without the protection afforded by the fuse 106, the pn junction of the transistor 102 would conduct as much current as the voltage source 104 could provide. This large current may damage the transistor 102.
Instead, the first diode 108 is now forward biased and conducts as much current as the voltage source 104 will allow. The large amount of current quickly blows the fuse 106, effectively disconnecting the voltage source 104 from the remainder of the circuit. The fuse 106 must be replaced once the polarity of the voltage source 104 is corrected, a disadvantage of this approach in terms of replacement costs and labor.
Referring now to FIG. 2A, an electrical schematic of a circuit 140 incorporating a PMOS-based reverse polarity protection system according to the prior art is presented. This circuit includes first and second transistors, 142 and 144, respectively. In this implementation the first transistor 142 is a p-channel MOSFET and the second transistor 144 is an n-channel MOSFET, each having a gate, source, drain, and body, although other transistor types may be used. A first terminal of a voltage source 146 communicates with the drain of the first transistor 142. The gate of the first transistor 142 communicates with a second terminal of the voltage source 146. The source and the body of the first transistor 142 communicate with a first terminal of an inductor 148. A second terminal of the inductor 148 communicates with a positive terminal of a diode 150 and with the drain of the second transistor 144. A negative terminal of the diode 150 communicates with a first terminal of a capacitor 152. A second terminal of the capacitor 152 and the source and the body of the second transistor 144 communicate with the second terminal of the voltage source 146.
In normal operation, the gate-to-source voltage (VGS) of the first transistor 142 is less than zero, and the first transistor 142 conducts current. Because the voltage drop of the first transistor 142 is interposed between the first terminal of the voltage source 146 and the first terminal of the inductor 148, the possible voltage that can be applied across the capacitor 152 is decreased. In addition, all current flows through the first transistor 142, regardless of whether it then flows through the second transistor 144 or the diode 150. This requires the use of a more expensive transistor.
Referring now to FIG. 2B, an electrical schematic 160 depicts the circuit of FIG. 2A when the voltage source 146 is connected with reverse polarity. To make operation more apparent, the circuit has been redrawn upside down. The gate of the first transistor 142 is now connected to the supply voltage. The voltage at the source of the first transistor 142 can be no greater than the supply voltage, so VGS≧0. The first transistor 142 is thus turned off, and no current will flow in the circuit.